The reflow soldering method (also referred to below as the reflow method) is particularly suitable for soldering of electronic parts. The reflow method is a method in which a solder paste comprising a solder powder and a flux is applied to necessary locations of a printed circuit board (which typically have lands made of copper) by printing or discharging through a dispenser, and after electronic parts are mounted on the coated portions of the board, the board is heated in a heating apparatus called a reflow furnace to a temperature sufficient to melt the solder powder in the solder paste, thereby soldering the electronic parts to the printed circuit board.
The reflow method can not only perform soldering at a large number of locations in a single operation, but solder does not adhere to unnecessary locations, so it can perform soldering without forming bridges even with respect to electronic parts having a narrow pitch. In addition, the solder paste can temporarily secure the electronic parts, so it is not necessary to secure electronic parts with pins, and the solder paste contains a flux, so an operation of applying flux is unnecessary. Therefore, the reflow method has the advantages that it can perform soldering with excellent productivity and reliability, and it can easily cope with decreases in the size and increases in the density of electronic parts.
A conventional solder paste which is used in the reflow method has been prepared from a powder of a Pb—Sn alloy, a typical solder alloy which has been used from antiquity. A Pb—Sn alloy has a low melting point of 183° C. for the eutectic composition (Pb-63Sn), so it has little undesired thermal effect on electronic parts which are not resistant to heat. In addition, a Pb—Sn alloy has excellent solderability, and it causes little occurrence of soldering defects such as unsoldered portions and dewetting. However, out of concern for the toxicity of Pb, in the electronic equipment industry, there is a strong demand for so-called lead-free solder which does not contain Pb.
Typical lead-free solders are Sn-based solders containing Sn as a main component. Lead-free solders which are presently used include binary alloys such as Sn-3.5Ag (melting point of 221° C.), Sn-0.7Cu (melting point of 227° C.), Sn-9Zn (melting point of 199° C.), and Sn-58Bi (melting point of 139° C.), and these alloys to which one or more third elements such as Ag, Cu, Zn, Bi, In, Sb, Ni, Cr, Co, Fe, Mn, P, Ge, and Ga are added. These alloys can be collectively referred to as Sn—Ag, Sn—Cu, Sn—Bi, or Sn—Zn based alloys.
The term “based alloy” used herein includes the alloy itself and an alloy further containing one or more other elements. For example, a Sn—Zn based alloy includes Sn—Zn binary alloys and Sn—Zn alloys further containing at least one third element. Similarly, a Sn—Ag based alloy includes Sn—Ag binary alloys and Sn—Ag alloys further containing at least one third element.
A Sn—Ag based lead-free solder and a Sn—Cu based lead-free solder have a melting point of at least 220° C. even for a eutectic composition of a Sn—Ag or Sn—Cu alloy. Therefore, when these are formed into a solder paste and used in the reflow method, the peak temperature at the time of reflow soldering becomes at least 250° C., at which temperature there is the possibility of thermal damage to electronic parts and printed circuit boards.
A Sn—Bi based lead-free solder has a low melting point near 139° C. at the eutectic composition of a Sn—Bi alloy, and when it is used as a solder paste in the reflow method, the peak temperature is at most 200° C., so thermal effects on electronic parts and printed circuit boards are avoided. However, a lead-free solder containing a large amount of Bi has a melting point which is too low, so it has problems with respect to heat resistance. Namely, when the interior of the case of a piece of electronic equipment reaches a high temperature during use due to heat generated by coils, power transistors, and the like, soldered portions of a printed circuit board which is soldered using this lead-free solder have a decrease in bonding strength, and there is the possibility of peeling occurring. In addition, a lead-free solder containing a large amount of Bi, which is brittle, has another drawback that soldered joints can easily peel off when they receive even a small impact.
A Sn—Zn based lead-free solder has a melting point of 199° C. for a eutectic composition of a Sn—Zn alloy. This melting point is close to the melting point of a conventional Pb—Sn eutectic solder, so when a Sn—Zn based lead-free solder is formed into a solder paste and used in the reflow method, the peak temperature can be made 250° C. or less, and there is little thermal effect on electronic parts and printed circuit boards. However, a solder paste using a Sn-9Zn eutectic alloy has poor solderability with respect to portions to be soldered made of copper, so soldering defects may occur such as unsoldered portions, where solder does not adhere, or dewetting, in which portions are wet by solder but repel the solder. Such soldering defects not only reduce the bonding strength but worsen the external appearance.
In addition, when a long period of time has passed after soldering of copper lands of printed circuit boards or copper leads with a solder paste using a Sn-9Zn eutectic alloy, there are cases in which solder peels from the interface with copper foil or copper lands due to corrosion. Peeling of soldered joints is a cause of malfunctions in electronic equipment.
Furthermore, tombstoning in which minute chip parts stand up at the time of soldering can easily occur with a solder paste using a Sn-9Zn eutectic alloy. If tombstoning occurs on a printed circuit board, electronic equipment in which the printed circuit board is incorporated cannot function at all.
In order to reduce or eliminate the above-described problems of a Sn-9Zn eutectic alloy, various Sn—Zn based lead-free solders to which a third element is added have been proposed. For example, a solder paste using a lead-free solder to which Bi is added in order to improve solderability such as one having a composition of Sn-8Zn-3Bi or Sn-8Zn-3Bi-0.1 Ag is known in the art. In addition, a lead-free solder in which corrosion resistance is improved by adding Ag and/or Cu to an alloy having a composition close to the Sn-9Zn eutectic composition is disclosed in JP-A 9-94687.
A Sn—Zn based lead-free solder to which one or more of Bi, Ag, and Cu are added does indeed have the effect of improving solderability and resistance to peeling compared to a Sn-9Zn eutectic alloy when used in a solder paste in the reflow method. However, soldering defects and tombstoning can still occur.
Soldering defects and tombstoning occur with a Sn—Zn based lead-free solder because a Sn—Zn based alloy is a powder of only one type of alloy. This alloy powder becomes rounded at the start of melting, and the molten solder does not further wet and spread, and this causes soldering defects. In addition, this alloy powder melts within a short length of time during melting. Therefore, if the temperature at both ends of a chip is different at the time of reflow soldering (such as due to a temperature gradient or a change in the temperature within a furnace), the alloy on the end which melts first pulls on the chip by surface tension and tombstoning takes place.
In order to prevent soldering defects and tombstoning with a solder paste using a Sn—Zn based lead-free solder, a solder paste which uses a mixture of at least two lead-free solder powders (a powder mixture) having different compositions (different melting points) is disclosed in JP-A 9-277082. A solder paste using a mixture of at least two alloys each having a eutectic composition is disclosed in JP-A 11-138282, and a solder paste comprising a Sn—Ag alloy powder mixed with another alloy powder for improving wettability is disclosed in JP-A 9-295182.
With a solder paste using a powder mixture of at least two types of solder powder, the alloy powder having the lower melting point melts first, and the alloy powder having the higher melting point is present in its periphery, so the alloy powder having the lower melting point which melts first does not ball up, it adheres to the portion being soldered in this state, and good soldering is carried out. In a solder paste using a powder mixture, the alloy powder having a lower melting point melts first, and after a while, the alloy powder having the higher melting point melts. Accordingly, it takes time for the alloy powder having the higher melting point to completely melt, and during this period, even at the other end, the alloy powder having the lower melting point begins to melt, so tombstoning does not take place.
However, a solder paste using a powder mixture of conventional Sn—Zn based lead-free solders has the problem that minute solder balls are generated at portions being soldered at the time of reflow. In addition, with these solder pastes, the problem of peeling from portions being soldered made of copper after the passage of a long period of time due to corrosion remains unresolved.
Accordingly, there is still a need for a lead-free solder paste of a Sn—Zn based alloy which has good corrosion resistance and which does not produce minute solder balls at the time of reflow.